Unfortunately, it seems your example doesn't match observed behavior. Consider:
Zone Test/Transition1970 2:00 - XST 1970 Jan 1 0:00u
3:00 - YSThttps://github.com/eggert/tz/commit/8973320770f676c249229c5c4ca402057e7ec098 says "a
loyear of 1970 includes a transition occurring at the very start of 1970 but a
hiyear of 1970 excludes the transition." The opposite behavior is observed:
$ ./zdump.exe -Vc 1969,1970 Test/Transition1970
Test/Transition1970 Wed Dec 31 23:59:59 1969 UT = Thu Jan 1 01:59:59 1970 YST isdst=0
Test/Transition1970 Thu Jan 1 00:00:00 1970 UT = Thu Jan 1 03:00:00 1970 YST isdst=0
$ ./zdump.exe -Vc 1970,1971 Test/Transition1970If the transition is one second later, the behavior matches the description. Currently zdump is inclusive then exclusive at a year-level, but exclusive then inclusive at a second-level. This is why I described the current behavior as "inclusive of the start year except exclusive of its first instant, and
exclusive of the end year except inclusive of its first instant". Since this wording is quite awkward, it may be simpler to adjust the code to match the description you provided.